The resolution of the conventional optical microscopy is limited by the wavelength of the visible light. Furthermore, at the highest resolution the conventional optical microscopy has a very shallow depth of field. These two limitations have led to the increased popularity of charged particle devices for the examination of specimen. Compared to optical light accelerated charged particles, for example electrons, do exhibit a shorter wavelength, which leads to an increased resolution power. Accordingly, charged particle beams, especially electron beams, are used in a variety of ways in biology, medicine, the materials sciences, and lithography. Examples include the diagnosis of human, animal, and plant diseases, visualization of sub cellular components and structures such as DNA, determination of the structure of composite materials, thin films, and ceramics, or the inspection of masks and wafers used in semiconductor technology.
Furthermore, charged particle devices are well suited for the examination of the microstructure of solid surfaces. Especially, the scanning electron microscope is a versatile instrument for examining the microstructure of a surface, because it combines high spatial resolution with depth of field in the same image, and requires only minimal sample preparation. Modern instruments distinguish features as small as 1 nm, while retaining crisp focus throughout tens of microns in the vertical direction. Hence, it is well suited for routine inspections of the intricate surface details of highly integrated circuits. Charged particle devices may, for example, be used in order to monitor the quality of the wafer processing in the semiconductor industry. Thereby, the device is actually located within the production environment, so that problems of the wafer processing are recognized as soon as possible.
However, conventional charged particle devices are not capable of providing accurate critical dimension, accurate height or accurate edge width measurements without the need of massive manual interference. In order to measure, for example, the height difference between two image points, usually two images are recorded with a defined specimen tilt between the exposures. However, mechanically tilting the specimen leads to a number of disadvantages. Due to mechanical imperfections a lateral movement of the specimen is inevitable which often results in misregistrations between the elements of a stereo image pair. Accordingly, additional alignments are necessary which slow down the process considerably. Furthermore, tilting large specimen, for example a 12 inch semiconductor wafer, requires a very robust and costly mechanical configuration in order to guarantee an adequate resistance of such a stage against vibrations.
In order to overcome the problems connected with a mechanical tilt of the specimen, it has been proposed to tilt the electron beam electrically in the electron-optical column to procure the same result, see e.g. B. C. Brenton et al. “A DYNAMIC REAL TIME 3-D MEASUREMENT TECHNIQUE FOR IC INSPECTION”, Microelectronic Engineering 5 (1986) 541-545, North Holland or J. T. L. Thong et al. “In Situ Topography Measurement in the SEM”, SCANNING Vol. 14, 65-72 (1992), FAMS, Inc. However, the height resolution of the proposed systems lies in the range of 75 to 100 nm, which is not sufficient for the requirements of the semiconductor industry.
Due to these problems, critical dimension measurements and side wall profiling are often done with an atomic force microscope. However, using an atomic force microscope requires an additional experimental setup which increases the costs significantly and is also very slow. Accordingly, there is a need for a faster and more automated method of examining a specimen which allows accurate critical dimension, accurate height or accurate edge width measurements.